Most of the radio colloids known in the art require the presence of a stabilizing material, usually a macromolecule, to prevent adherence of the colloid to the walls of its container and agglomeration of the colloid particles into macroaggregates. While gelatin and dextran have been used for this purpose in the past, occasional adverse reactions have been associated with them. The use of human serum albumin as a macromolecular stabilizer for a 99m-technetiumtin(II) colloid the particle size of which was stabilized in a range suitable for reticuloendothelial organ imaging is disclosed in an article entitled "A Simple `Kit` Method for the Preparation of a Technetium-tin(II) Colloid and a Study of its Properties" by Max S. Lin and H. Saul Winchell in Journal of Nuclear Medicine Vol. 13, No. 1, pages 58-65. These macroaggregate stabilizers are essential to such preparations as they will tend to coagulate and settle if the stabilizers are omitted therefrom. In accordance with the present invention it has been found that stable, efficiently labeled technetium-tin (II) colloid having a particle size suitable for reticuloendothelial imaging can be prepared without the use of the macroaggregate stabilizers taught by the prior art.